Linear motor



Clifford' JQ'I'I'Ims, wlmi-.md Inns, Calif., assigner-t0 1'.

rporation, Culver fig', C alif.`a corporation of Delaware 'Flied Feb. 9,1968, Ser. No. 704,291

` Int. C I. H02l 41/ 00 ABSTRACT THE DISCLOSURE 'A linear motor capable of providing areasonably long stroke and rapid acceleration. The motor is comprised of a core structure defining an air gap around acentral leg.-

Means arel provided for developing a substantially. -unitrieally v4disposedaround the central leg with the turns thercofthreading the gap so that a eurrentdriven through `In accordance with a significant feature ofrthe invclil- ,A .l l

tion, in order to minimize the'inductance of the drivecqil to permit rapid' current changes, abucking coil is also 14 Claims Afield produced by the drive coil.

an alternative embodiment of the invention, the bucking form magneticeld through the gap. A substantially rigid drive coil structure is conccntrically-disposed around the central leg withlthe turns thereof threading the'vgap. The coil is mounted for reci procal'movement along the'central leg in response to a propelling force developed on Vthecoil by driving a current therethrough. In order to mnimize induc'tance,a bucking coil is also wound around thej central leg and connected in series opposition to the movable drive coil.- Y

BACKGROUND OFTIIEINVENI'ION Y fielder the inventori t f The present invention relates toelectric motors capable of providing linearmoveme'nt.,

More particularly, the present invention relates to .linear t motors suitable for use in applications where high'specd, accuracy and relatively long stro-kes are required. One

such application is as a linear positioncr in a magneticdisc memory. Such memories employ magnetic discs which may have as many as twelve hundredconcentric tracks recorded on a surface havinga twelve inch radius. In]l such memories, a head carrying arm is-provided adjacent each disc surface. The arm may,` for example, carry only four heads so that it is necessary to be able to move the arm radially three inches with respect to thedisc in order to position a head adjacent to a selected track. Itl will beappreciated that such applications require exremely accu-rate positioning resolutions. Moreover, inasmuch as the head positioning time constitutes a significant portion of the overall memory access time, it will also .be appreciated that rapid positioning is' extremely ,important. A further requirement of a linear'positioner for use in a discA memorysystem is that it have a relatively wound around the central legand connected to the drivecoil 'soasto minimize thejluic in the central leg. Anotherthe coil will develop a 'propelling foreeon. thc oil Smm-- 1 ture suilicient to -moveit along the-central `leg.".`

significant feature resulting from the introduction of the 'vf bucking cO-il is the reduction in the net external-magnetic In accordance with a preferred embodiment of the invention, the bucking coil is `fixeclly mounted and disposed I concentrically around the central leg. In accordance with coil can also be mounted for movement along the central 'l leg.. A

In accordance with astill further aspect of the present invention. embodiments of the invention can b e utilizedas lineartachometers in which a moving carriage carries 'a sense coil (corresponding to the' motor drive coil) through the garito inducea voltage thereacross. The use of a bucking coil in such embodiments reduces the sen.-

vsitivity of the tachometer to external magnetic field s.`

` lhe'- novel features of the invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when!A read in conjunction with 4theaccompanying drawings.

BRIEF DESCRIPTION OE'THE DRAWINGS 'FIGURE l is a perspective viewo'f alinear positioner for use in a magnetic'disc memory system which employs.

a lineai motor in accordancwithjthe present invention;

FIG. '2 is a vertical sectional view taken through the` linear motor ofFlG. l;

FIG. 3 is a vertical sectional view taken substantially along the-plane 3-3of FIG'. 2;. v

FIG. 4 is a schematic diagram illustrating onetforrn of electrical interconnection between the movable. drive coil and stationary bucking coil ofthe motors of FIGS.

` form of interconnection betweenthe movable drive coil long stroke, e.g. more than one inch, in order to minimize. i

the number of heads required per dsc'surface.

I Description of the priorurt The prior art discloses many linear positioning devices` intended for use in magneticv disc memory systems; eig.,

vsee U.S. Patent No. 3,134,880 andU.S. Patent No. 3,314,057. Although` such prior art devices may function adequately in many types of disc memories, they graduallyf become unsatisfactory as track density' requirements increase and positioning time requirements decrease.

SUMMARY OF THE INVENTION In view of the foregoing, it is an Object of the present invention to'provide a fast and-accurate linear motor capable of providing a reasonably long stroke.

In accordance with thepresent invention, a motor is providedI which includes a magnetic core structure de- I fining-an airgap around acentral leg. Means are provided for developing asubstantially uniform. magnetic: field through the gap. A fnovable drive coil is concen- FIG. 5 is a' schematic diagram illustrating an alternate and statiOnary'bucking-coil of che motor of FIGS. 1 3;

" FIG. 6 is a vertical sectional view illustrating a furtherl embodiment of the invention;

FIG. 7 is a schematic representation Ofa further embodiment oft-he invention; and

' FIG. 8 is a schematic representation of'astill further embodiment of the invention.

DESCRIPTION oI`= 'rHE PREFERRED 4` EMBODIMENTS r Attention is now called to FIG. l which illustrates linear positioner in accordance with the present invention.' Although the linear positione'of PIG. 1' isirttended pri marily to be utilized for positioningmagnetic headsin a disc memory system.. it will be'readily recognized that the'apparatus is suitable for use in many other applications. Y The linear positioner of FIG. 1. includes a linear motor 1 2 capable of driving a rigid'ea'rriage structure 14. The' carriage structure can be provided with tracks 15.adapted to ride in mating channels or ball bearings (notshown) to constrain the carriage movement toa linear direction.

The linear positioner-oftEIG. l also :includes'a' linear tachorneter I5, which, as will be seen Z'Iereinaftier,ope'r-v i' ates upon substantially the same principles as linear motor lf2. The motor 12 vand tachometergljare supported on ay suitable base 1 8.

' The motor 12 isfcomprised of a soft iron 'core struc-ture i 20. The core structure 20; as is best shown vin FIGS. 2-

'and 3, maybe formed--from'two oppolsitelyorien'ted rshaped portions 22A and 22B. Core structure portion 22A j l includesayertical leg 24A, an upper leg26A,'a :central f leg 28A.'v and a lower leg 30A.' Similarl'y,` core-structure vportion 22B has a vertical leg 24B, .an'upper leg` 26, a

v. central leg 28B, and a lower leg 30B.'Upper passages 32A` dimensional' the coil 5 0 can be pproitimately orte-l'ralLV the longitudinal Adimension ofthe centr-al leg 28. Thug.

and 32B respectively space 4the upper legs 26A and 26B' 'I from the central legs 28A and 28B. Similarly, lower pas-` sages 34A and 34B respectively space the lowerl legs 30AA and 30B from the central legs 28A-'and 28B.

The core structure port-ions 22A and 22B are oriented with respect Ato each other-'as shown in FIG. 2 with the faces of the free ends of the upper; central, and lower legs in intimate contact with each othenljlereinafter, thecompositecore structure 20 will be referred to as being comprised of an upper leg 26, a central leg 28, and a 20 will be referred to respectively by the numerals 32 and 34. t It will be appreciated that although the Acore structure 20 has been illustrated in FIGS. l and 2 as being comprised of two E-shaped portions, it could in fact be comprised of a lesser or greater number of portions depending upon the manufacturing techniques selected,

- flower leg 30, and vertical legs 24A and 24B. The upper -and lower passages through the composite core structure In laccordance withthe present invention, means are" provided for establishing a magnetic field vin the passages 32 and 34 which'extend substantially parallel to the Avertical legs v24A and 24B. In accordance with the pre.

ferred embodiment ofthe invention, permanent magnets .v

36 are secured to th'e uncler`sidev of the upper leg -26 are secured tothe upperside of the lower leg 30 within wit'hin'thc passage 32. Similarly, permanent magnets 38A 'i the passage 34. The jvertical dimensions (as shownin FIG. 2) of vthcpermanent magnets 36 and 38 are less than the vertical dimensions of the passages 32 and'34 to thereby respectively define gaps 40 and 4 2. That is, gap

`40`is delined'between the permanent magnets 36 andthe central leg 28,' and gap 42 is defined between the perm;

'anent magnets 38 and the central leg 28. The permanent magnets 36 and 3 8 are oriented so as to create magnetic fields extending either into or out of the central leg 28.

` The dotted lines 44 and 46 in FIG. 2 represent magnetic tlux lines, which'originate at thegpermanent magnets and lextend into the central leg-28, and then through thel vertical leg 24A'toveither the upper leg 26 or lower leg 30. As will be better appreciated hereinaften'a linear motor in accordance with the present invention willisatisfactorily operate if the magnetic fieldsboth extend lin an opposite direction, that is,` from the central leg 28 across the gaps to the'permanentmagnet. In accordance with the invention, a substantially rigid multim-rn drive coil is wound on a coil form 51 concentrically disposed 'around the central leg 28. The drive coil 50 and form 51 together form a rigid structure which 'secured between a pair of carriage side frame members 52 and 54. The carriage side frame members'52 and-54 may be formed or' a variety o f materials which are of `a size and shape enabling them to be-light in weight but stiff. The load to be driven may be connected to theA carriage 14 opposite the drive-coil 50 end,.Current is conducted to the movable coils through exing members 56. Only oneof the tle'xingmembersis illustrated in FIG. 1.rv The flexing members 56 leave a first lend 5 8 anchored to but insulated from the base 18. A second end 60 is secured tobut insulated 'from' a side frames;v

for example,` if the longitudinal dimension ojthe central leg is foul' inches, the coil 50`can have a longitudinalj 'i :dimension of two" inches with the dill'erenceV (two inches) constituting .the stroke length.

ln orderV to physically motivate the drive coil 50 and carriage 14 rigidly enupledA theretoanje'lectrical currentV is driven through the drive coil which interacts with the magnetic feldthiough the gaps 40 and 42 to developi va force on thel coil 4structure which acts parallel tothe' longitudinal dimension of the centralleg A28. In accordance with the .preferred embodiment'of the invention, the'supporting leafr springs l5 6 are electricallly conductive l and a source of potential is connected thereacross to drive a current through the drive coil 50. The explanation of the electrical connections between 'the leaf springs 56, the

drive coil 50, and a bucking coil to be introducedwill" vbe discussed subsequently in conjunction with the eitplana tion 0f FIGS. 4 and 5, Y

lt has previously been pointed out that in order to be useful in the contemplated applications, the motor shouldv for the motor to be fast, the lateral dimension of the gaps 40 and 42 should be large, since the magnitude 'of the I A i force developed on the coil 4strt'tcturc is substantially proportonal to the lengtheff'conductor of 'coil 5 0 within the gaps. In order to maximize the magnetic field intensity through the gap, theverticaljgap dimension should be as smallas possible. 4/lllicpughthe embodiment o f the inventionthus far deseribedwill operate as alinear motor, 4 it Vmay not be quite fast enough `to satisfycertain application requirements` I t Vshould' befp'preciated that response he rise time of the leading edge of the drive coiltzurrent;v That is, ifthe drive coil `speed is directly related to t current increases to'its rated value very quickly, the

physical response of the drivecoil structure will be very if the drive coil current rise' drive .coil struc-l rapid. On the other hand,

time is slow, the physical response of' the Ature will be lcorrespondingly slow.

In order to enable the rise time ofthe drive c oil current i to be very rapid, it is necessary to minimize the drive coil inductance. Unfortunately, this requirement is nconsistent with the motor structure thus far recited because the drive coil will establish ux'in the central leg 28e,A thereby causing the drive coil inductanc'e to be larger than if thecentrallegwere absent. I n addition, the fluit `establishedby-the drive current might saturate some parts of the magnetic path, especially near the ends of the central leg. Another reasonv f or desiring inductance to be minimized is to reduce thc'net external magnetic lield' set up by the drive coil. l In accordance withta significant feature of the present invention, in order to minimize the drive coil inductance,

. a bucking Vcoil is wound around the'central leg 28. The" The'stationan-Vbucking lcoil 70 may be wound along the f entire lengthof-the central leg 28 with the same pitch member of the carriage 14. 'Ihe"characterstics of the.l

flexing members are `selected to provitlefa low resistance connection to the drive c'il'and'to-provide a negligible' loading effect on the motion of the carriage.

.In order for the drive coil 'V50 tojbe'movablealong the i' central leg 28, its length, of course', thirst' be shorter .than the length or longitudin'nal dimension of the centralfleg as that of thefmvable kdrive coil 50. As shown in FIG.,

4, one ,end-'of the movable. drive coil 50 can be connected toone ofwthe lead springse561. The second end of the ,drivecoil 50 .c an be connected toa movable contact or brush `7 2,' which-.is ganged with a second brush 74; The; brushes 72 and -7-1, respectively, contact insulation-free areas of the l stationary bucking coil'70. The vbrush '7 4 is electrically connected to'A the second leaf spring 563.

and 2; the longitudinal be very fast, andhave a reasonablylong stroke. In order.A i

" 'current source is intended through the coils represented by lh 'The movable brushes 72 and 74 are carried by coil; fr form 51 and are positioned so that` for any positionof the movable coil 50, a correspondingportion of the stationary bucking coil 70 will beenergized That ,is to say,

ciples as 'the linear motor 12.More particularly,

freeends' of' springs 561 and 562 Ato the flux in the portion of the central leg 28 surrounded by` the movable coil 50 will always "be minimized by the combined ezect ofthe coil 50 and that portonof `the 'c oil 70 selected by the' movable contacts 72 and-74. As a active portion ofthe coil 70 producing opposite effects, the next fix produced by the two coils in the center' leg will be very much reduced over what either alone would produce. In fact, theinconsequence ofthe coil SO-and ductance can be made substantially less than theaircore inductance of the movable coil.

Although the arrangement showe in FIG. 4 yieldsex`. cellent results, it requires the utilizationof brushes which are sometimes objectionable becausel of cost, friction, space, and maintenance requirements. An alternate solution which does not'require brushes andiwhich is normally-quite acceptable froml a performance standpoint, is to place the same number of'tum's on the stationary coil as are on the drive coil, but to spread them out over perhaps twice the length of the drive coil. That is, as shownin FIG. 5, one end of the drive coil connected to leaf spring 561. The

e'nd of the. leaf spring 562 is connected to the terminal 78 of the stationary coil 70 in FIG. 5. Whereas a current source was .intended to be connected between theeleaf springs 561 and 562 in FIG. 4, -in the embodiment of FIG.

5, it isconnected between the leaf spring 561 and terminal 80 of the stationary coil 70 to thus drive a current through coils 50 and 70 in the direction of the arrows. The arrangement in FIG. 5 results in a motor with a larger terminal inductance than that illustrated in FIG.

.,4 but which is still low enough to be acceptable for -many applications.

It has previously been pointed out that in order to 'develop the maximum force on the movable coil, as high a magnetic ux'concentration as is possible should be developed in the gap, and as much of the drive coil conductor as is possible should be disposed within the gap.Y

FIG, 6 illustrtesa cross section of an alternative embodiment of the invention which is designed -to maximize the fluxconcentration and percentage of the 4drive coil vwithin 'the gap. In the embodiment of FIG. 6, theupper. compositeleg 80 `is comprised of a plate of' iron 82 disposed on top of a permanent magnet assembly 84. An

additional iron member 86 is disposed beneath the magtapered sides By utilizing the cylindrical central leg 92 `of FIG. y6, a greater percentage of the coil conductor is disposed withl -iun thegap. Additionally, by shaping the iron members 86 as shown in FIGJG, the ux concentration within the gap is maximized. These two effects together assure that` a 6 maitimumiorce is developed on the drive coil which in turn assures high acceleration of the carriage.

Attention is now again called to FIG. l and, more v particularly, to .the .linear ltachometer 416. In 'certain applications of linear positioning devices, it is desirable 'to 7 .monitor the velocity of thecarriagel in, order, for exf' ample, to determine when it has reached zerofThe'linear tachometer 16 of FIG. l 'is capabeof performing this function and "operates upon substantially the same prnthe 50 is again' second endof the drive coil 50 is connected to the leaf spring 562. The anchored taehomete'r`16 includes a core of upper and lower leg's. 112 and v114 andvertical legs' .116 and 11-8. The 4legs'define '5. permanent magnetsy 122' 'the magnet assembly 122 and the `leg l12.

A sense coil "124`(cor`resp0nding to the drive "coil in .motor embodiments) loosely envelops leg 112. [he 'coil 1-24 is secured to stucl 126 of the carriage 14'. A stationary directly on the leg 112 vand as to developsan oppositely directed field through'leg 112, In the operation of theA i moved linearly in response motor drive coilstructure. the turns of the 'sense coii.124 will cut the lflux lines bucking coil 128 is wound is connected to the coil 124 so tachometer, .as the carriage is 15 to the force developed on the within the gap 121, thereby generating a voltage in the coil l24 which is relatedto the linear velocity of ihe coil 124 and carriage 14. By age provided 'oy the coil 124, the velocity of the carriage- 14 willbe known. It is pointed out that a-feature of the tachometer Vconstruction as shown in FIG. l is thatthe 'output voltage Adeveloped on vthe sense coil 124 will tbe insensitive toany lateral movement of the coil 124v or 25 in other wordsfthe outputw'oltage will be'related only to longitudinalmotion' along the lieg 112. Utilization of the fields sincesuch fields would induce opposite effects in out.

ly illustrates a still further embodiment ofthe invention which makes more efficient vuse of the fields set up by theis-illspermanent magnets.- Altho'ugh, sa. linear motor trated in FIG. it wili be. appreciated that thefeatures introduced therein canniso be utilized in tachonteter etnbodiments of the`inventionl Briefly, the concept introb y duced inthe embodiment of-FIG. 7 is to`form a gap'in f one of thevcrtical legstekg. leg 24B of FIG. 2) of the `core structure and to utilize the return flux therethrough bp providing a movable bucking coil within the gap as an auxiliary drive coil.

More particularly, the-embodiment vof FIG.. 7 utilizes` underside of leg 154 in passage 160 and the upper side of-i 158 in passage 162 thus defining gaps 168 an d 170..

les

I The core structure 150 thus far recited is idcnticalto the ly, in lieu of using-a right vertical leg to bridge `-legs 154, 15-6 and 158, pole pieces 172 and 174 are provided which respectively project toward central leg 156 but define gaps 176 and 178 therebetween. It should be readily appreciated that the magnets 164 will establish oppositely www, magnets 166 will establishvopp'ositely directed ux are directed away from central leg-156.

tral leg 156 threading gaps 168 and 170. A bucking coll .182, having a winding sense opposite to coil 180, also concentrically wound around'central leg 156. Thedrive coilg180 and bucking coil 18?. are electrically connected if in series. Additionally, the coils 180 and182 are vformed s structure4 comprised E.'

.a passageextendngtherethrough. Means for'creating a magnetic field such as are disposed within die 'passage 120 to establish a magnet-ic field, in `the gap 121 between" monitoring the output volt the windings 124 and 128 and thereby cancel each other u 'Attention' is now' callcdto FIG. 7 which diagrammatical-l 5 v core structure 2.0 ofj'FIGfZrHItdifi'ers therefrom how-. i 2 everfin that the upper central 'and lower legsare longer and'extend further from the left vertical leg. Additionaldirected flux lines-through the gaps V168 and 176 as, for example, are represented by the dotted arrow lines. Simies through gaps and 178 as shown by the dotted i ,arrow lines. It will be 'noted that the ux lnest hrougl 1 the gaps 168 and 1'70 are' directed toward the centrali A drive coil is-concentrically wound around cen- I I movement along the central leg 156.

It willbe apparent that'by driving acurnent through" serially connected coils 4180 'and 1.82, forces'will be `developed on both of the coils which act in the same direction alongv centralleg l.156. For example, ifa current is" driven through the coils in the-direction ofthe arrows,

afpropcllirig 'force to the right will'be developed onboth coils 180 and 182. Thus, the force developed on the bucking coil 182 aids the force developed on drive lcoil 180.

Nevertheless, the two coilsin series willfdeline a'lower,

inductance'than either v'one' alone. -liloreover, the coils will develop oppoistely directed fields in the centrall leg 'i 1S6'and will therefore both prevent saturation therein and minimize the generation of external eldst f Although the. utilization of the bucking coil in FIG. 7

yields a net inductancc lowerl than the inductance which would be provided bythe drive coil alone, it still may not be low enough for certain applications. In ordcr'to lower the` inductance further while retaining th'e use of the re turn path gaps 176 and 178, a pair ofaddinonal stationary.bucki ng coils can be concentrically wound on the central leg 156 as shown in FIG., 8'. More. pariicularly,

inthe embodiment of FIG. 8, a stationary bucking coil 184 is wound on the central leg 156 immediately beneath the drive coil 180. The coil 184 is wound opposite to the coil 180. Additionally, a stationary bucking coil 186 is wound on th'e central leg 156 beneath the movable b uclting coil 18 2. The winding sense of coil 186 is opposite to that of coil 182. The coils 184 and 186 can be spread out over the path length of coilsv180 and l8 2 respectively in' the manner previously discussed in conjunction with FIG. 5 or alte'matively a-rnoving brush arrangement can be used of the type previously discussed "in conjunction with FlG.- 4. Thatis, by' utilizing moving brushes, the

portion of each of the stationary coils immediately adjacent to the movable coils for any position thereof can be energized to achieve optimum bucking.

From the foregoing, it should be appreciated that sev eral embodiments of linear motor and tachometer corrstructions have been disclosed herein whichare characteristically veryfast and accurate and which are able to provide arelatively long stroke.

v The embodiments ofthe invention in which an errclufy i sive property or privilege is claimedare dened as follows:

1. vA linear motion device comprising:

.a core structure denin'ga 'gap having substantially perpendicular longitudinal, lateral, and vertical dis mensions;

a permanent magnethaving substantially perpendicular longitudinal, lateral, arid vertical dimensions and having parallel pole faces spaced by said magnet vertical dimensions, said magnet longitudinal and lateral dimensions being substantially equal vto said gaplongitudinal and lateral dimensions, respectively; means supporting said permanent magnet on said core structurefor establishing a magnetic lield across said gap extending substantially parallel to said gap vertical dimension and of substantially uniform interr-- sity along said gaplongit'rdinal dimensions;

- a rigid drive coil structure 'comprised of a plurality-of means supporting said dnve coil structure for recrprocaldimension is substantially larger than said gap vertical, i

dimension,

3. The .device of claim-1 including a second conductor said drive ooil.-

said lateral dimension; and means interconnecting said rst and 1 in said core:s truct'ure in of current t-:rs ai 1 comiluctors.4

A linear .motion device comprising: y a core structure' including a central leg 'and longitudinal, lateral, and vertical? dimensions;

dimension'of said gaps, said structure including a of said gaps;

permanent magnet means supported by'said core struc-' ture establishing magnetic elds across said upper and lower main gaps each extending parallel to said gap vertical dimensions with both fields extending. -either toward or away from said central leg, each.

'- of said fields being of substantially uniform intensity 'along the longitudinal dimension of said gap; and means for applying current to s'aid drive coil tothereby develop a force on said rigid structure tending to move it along said central leg. i

ing current to said drive coil includes first and second fiexure members, each having first and second ends;

means anchoring said tirst ends of said first and second flexure members relative to said core structure; andV means electrically coupling said second ends of said first and second lexure membersto said drive coil.

v6. The motor of claim 4 includinga stationary coil wound about-said central leg; and

means connecting said drive coil in series with said stationary coil sothata' current therethrough de velops oppositely directed magnetictields in -said central leg, i

7.' The motor of claim 6 wherein said stationary coil has va greater number of turns than said drive coil and Vwherein said turns of said stationary aid drive coils are 0f substantially the same pitch.

8. The motor of claim 7 wherein'said -mean-s connecting said drive coil to said stationary coil includes movable contact means for connecting substantiallythe same nun? ber of turns in said stationary coil in series with said drive coil as there are turns in said drive coil.

9'. The motor of claim 6 wherein said stationary coil i extends a vgreater distance'along said longitudinal dimension than does said drive coil.

10. The motor of claim 9 wherein the turns of said a greater pitch than the turns of stationary' coil have 11. A linear motiondevioe comprising:

a core structure includinga central leg 'and upper .and

lower legs spaced therefrom-'to respectively define upper andlower main gaps;-

a rigid structure supported for reciprocal movement'- along said central leg, said structure includinga drive coilconcentrically wound around said central lez and threading said upper and lower main gaps; means establishing magnetic fields across said upper and lower main gaps both extending either toward orv away from said central leg; means for applying current to said drive coil to thereby move it along said leg; and

pole pieces disposed onv corresponding ends o f said upper and lower legs projecting toward said central leg and defining upper and lower auxiliary gaps theref between;

elongated the :direction ofj;`4

second conductors t' A so as to develop oppositelypdirected magnctields 1 response to the application Upper andi; f lower legsspaced therefrom to .respectivelydefine parallel upper-and lower main gaps each having rigid structure supported for reciprocal movementalong said central leg parallel to said longitudinal drive coil concentrically wound around said central leg and threading said upper and lower main gaps,

the dimensions of said drive'coil along said central leg being smaller than the .longitudinal dimensions 5. The motor of claim 4 wherein said means for apply?.

develop a force on said rigid structure tending ttl-' -A series, and re'wound tlineadirlg.saidpper aid lower auxiliary gap's.'

12; Y'The linear motion' device. of claim V11 including means interconnecting said drive coil and-said auxiliary- V' 'drive coil so as lo develop -opposicly directed' magnetic "fields in s1id.central'leg,' l I 1-3. The linear motion device of one another.

claim 11 wherein said 'Y drive coil and said auxiliary drive coil are connected in `with an opposite sense relative tov .14. The linear motion device of claim 13 includingwiirst and second stationary bucking coils wound on vsaid centrai leg adjacent to said drive coil and said auxiliary drivcv V coil respectively, said first and secondA bucking coils respectively having winding senses opposite `to that of s aid drive and auxiliary drive coils; and

means'connecting said first and second bucking coils in series with said drive coil aid said auxiiiary drive coil.

' OTHER REFERENCES-ff i Us. c1. xx. 

